Intervertebral cages are well known. One known configuration is comprised of an endless, contiguous wall defining an interior space that is accessible from two opposite, open sides. The cage may be inserted between two adjacent vertebrae to maintain space between the vertebrae and promote fusion between the vertebrae. For example, the interior space may be filled with bone graft material or other biologic agents to facilitate boney fusion between the adjacent vertebrae.
Another known configuration includes two pieces that can be integrated to define a cage. An example of such a configuration is disclosed in Falahee, U.S. Patent Publication No. 2009/0030519, which alleges that devices according to the invention eliminate multiple steps, instruments, and trays, while being capable of a custom fit. The devices according to the invention permit easier and greater access to end plate surface area, and can be used with autografts, allografts, and biologics.
In U.S. patent application Ser. No. 12/834,855, now U.S. Pat. No. 8,491,658, the inventor of the present application has proposed a two piece cage design that includes a base member having three sidewalls and a closure member that is coupled to the base member to define a cage. As disclosed in U.S. patent application Ser. No. 12/834,855, a two-piece cage can include a generally elongated U-shaped base member having two opposing sidewalls each with a terminal, free end and a closure member that mates and preferably locks with the base member to define a cage. The base member and the closure member in a two-piece cage system includes mating parts that register with one another so that the two pieces may be assembled in-situ; i.e. during the operation.
A two-piece intervertebral cage can be made from a single material (e.g. a metal) or a composite material. The material selected for the cage will usually have a modulus of elasticity close to the modulus of elasticity of human bone and thus, relatively speaking, may be rather compliant. Thus, the base member in a two-piece cage system, which may be an elongated, U-shaped body, may be deformed when inserted into an intervertebral space especially when the base member is under pressure. Deformation of the base member is more pronounced when its length is longer than its width. Thus, the distance between the terminals, free ends of the sidewalls of the base member may vary when the base member is inserted in an intervertebral space.
In a lateral operation, depending on the type of retractor used, there tends to be some movement of the retractor in relation to the vertebral body and the disc. These movements and size limitations tend to cause a change in the shape of the base member also. There are limitations of exposure and size of possible annulotomy in the intervertebral disc annulus due to anatomical considerations such as location of neural structures, vessels, and muscle groups. In a lateral or anterolateral operation, the base member and the closure member of the cage are inserted through a long narrow retractor. Thus, the retractor makes the coupling of the closure member to the base member challenging in the presently available designs. The deformation of the base member of the cage, e.g. the change in the distance between the terminal free ends of the base member after it is inserted in place, causes a misalignment of the mating parts of the base member of the cage and its associated closure member making it more difficult for a surgeon to assemble the cage and complete the operation.